"pt" is "point" unit: not the dot or pixel point, but the font size unit...
Anyway, "em" is NOT equal to an absolute value (pixel/n) but is a unit relative to the current (related to context) target element font size: rouglhy, 1 em almost equals the current font width of "m" and/or line height requirement (few more than height of characters ^^)
see this page for more information
em=px/16 if you don't chnge font-size of parent.
px in mm pc are absolute units recommended for print.
em rem % vw vh and etc are relative units; recommended for screen.
vh and vw are 1% of width and height of viewport.
Pixels (px) are relative to the viewing device. For low-dpi devices, 1px is one device pixel (dot) of the display. For printers and high resolution screens 1px implies multiple device pixels.